Transmission system for sound-carrying currents



xaminer Reference D. B. HARRIS ET AL may 16 1933 TRANSMISSION SYSTEM FORSOUND CARRYING CURRENTS- Filed Oct. 20, 1936 Sheets-Sheet l 15 erfi om'ewewm Examiner 6 J 3 ,7 w I wn m p e 6 m 2: S3 W V H W m 3 K m a w 4 Imaha z: JWJ M m C D. B. HARRIS ET AL s T N E R R u C G N I v M O l o w 2s W at 00 F0 mm 9 T n HF S N 0 I s s I M s N A R T .QQQ'OQOOOO In D May16, 1933 txaminer e C n nu r rm 9 S S 0 r C May 16, 1933.

D. B. HARRIS ET AL TRANSMISSION SYSTEM FOR SOUND CARRYING CURRENTS 3Sheets-Sheet 3 Filed 001:. 20, 1950 Ji WWWM i Patented May 16, 1933UNITED STATES PATENT OFFICE DONALD B. HARRIS AND ORVILLE W. KNAUSS, OFST. PAUL, BUNN'ESOTA, ASSIGNOBS, BY DIRECT AND MESNE ASSIGNMENTS, '10PROGRAM SERVICE comm, 01' 8'1. PAUL, MINNESOTA, A CORPORATION 01DELAWARE I TRANSMISSION SYSTEM FOR SOUND-CARRYING GURBENTS Applicationfiled October 20, 1930. Serial No. 489,888.

number of distant stations, such as subscrib-' ers homes, and whereinconductors in con ventional communication systems could be convenientlyutilized to transmit not only the sound-carrying currents, but, inconjunction with ground, other elect-ro-motive forces for operatingamplifiers located at the distant stations.

As pointed out in our said patent, one of the important advantages ofthe original system was to eliminate interference caused by induction ofenergy into other conductors neighboring or in the same cable with theconductors utilized in our system, which induction is often referred toas cross talk. However, cross talk or induction was not entirelyovercome due to the fact that the line conductors of the original systemwere not balanced with respect to ground.

Another objection to the original system was that a small amount of lowfrequency regeneration resulted in the system due to a feed-back fromthe plate circuit to the grid circuit and through the medium of apotential generated along the plate circuit side of the line.

In our joint application entitled Transmission': .system forsound-carrying currents, S. N. 460,882, filed June 13th, 1930, wedisclosed an improved transmission system for sound-carrying andimage-carrying currents which lessened the said objectionable featuresof said original system. In said improved system, potentials formaintaining the grid at an operative potential with respect to thefilament element were supplied from the central ofiice through a circuitin- 59 eluding one of the line conductors in conunction with ground. Theline conductors of this system were adequately balanced with respect toground and efiicient means were provided for preventing undesirable re-5 generation effects. In the said improved system, under someconditions, however, noise induction of small magnitude resulted in theline transmitting the program service due to a difference between theground potentials at the central and the remote stations and due to theimpossibility of maintaming the terminating impedances in a condition ofperfect balance. The installation costs of said improved system werehigh due to the necessity of making a connection to ground at thesubscribers stations.

In both the ori inal and said improved systems, because of the necessityof ground connectlons in conjunction with the several circuits includedin the system' and dueto the high plate potentials required inconnection with the amplifier, some danger is presented to workmeninstalling or servicing neighboring telephone or communication lines,and the said high potentials may break down the protective devicesassociated with the conductors of our system at either the central ordistant stations.

It is an object of our present invention to provide an economical andimproved system of the class described utilizing apair of conductorsbetween a central and a distant station for transmission of sound andimage-carryin currents as well as grid and late potentia wherein theobjectionable eatures of said original system and said improved systemare obviated; wherein the system is simplified and may be installed atless cost; wherein sound or image-carrying currents are transmitted fromthe central station to the remote station with a minimum loss of energy,and wherein the pair of line conductors employed are isolated fromground thereby eliminating all chance of noise inductions.

It is a further object to provide a system of the class describedwherein conductors, such as, for example, a pair of conductors in aconventional communication cable may be utilized for transmittingsoundor imagecarrying currents from a central station to a distantstation; wherein the same conductors transmit electro-motive forces formaintaining the plate element of the amplifier at an operative potentialin respect to the filament and wherein potentials for maintaining thegrid element of the amplifier at an operative potential in respect tothe filament element are created in the remotely located amplifier bymeans of electro-motive forces generated across a resistance inserted inthe supply circuit, including the central oflice source of directcurrent and said line conductors. In our present invention all signalarlllg supply circuits may be isolated from on Another object is toprovide a s stem wherein the safety hazards previous y referred to areeliminated throughout the system.

These and other objects and advantages of the invention will be fullvset forth in the following description made in connection with theaccompany drawings, in which like reference characters refer to similarparts throughout the several views, and in which Fig. 1 is a diagram ofone form of our system, and

Figs. 2 and 3 are diagrams of other forms.

Referring to the first form of the invention, illustrated in Fig. 1, theelements and apparatus located at the left of the heavy broken line AAare located at the central station while the ap aratus at the right ofsaid line are located at one of the distant stations, such as the homeof a subscriber to the program service. At the central station a sourceof alternating current is available, from the output of a suitableamplifier or other apparatus. In the drawings, the plate element of thisamplifier is designated by the numeral 4. B battery B is also located atthe central station. Another source of direct current, such as a Beliminator or generator may be substituted for the B battery. Thecentral ofiice output transformer, comprising primary winding 1 andsecondary windings 2 and 3, having condenser 5 inter osed therebetween,is disposed between t e source of alternating current and lineconductors X and Y which extend between the central station and thedistant station. These line conductors may be a pair of wires in aconventional communication cable. The output transformer is adapted toreduce the impedance of the source to a point where the termination ofthe line is made equal to the characteristic impedance of the lineconductors. The B battery B is connected to the line conductors throu htransformer windings 2 and 3. Key 6 is also located at the centralofiice for the purpose of operating a remotely located relay at thereceiving amplifiers at the distant stations, the contacts of said keyopening and closing the filament circuit of the distant amplifier.

At the dist-anct station is located the thermionic am lifier andaccessory apparatus. The ampli er disclosed is of the single stagepush-pull type. It is to be understood, however, that an amplifier withmore than one stage or an amplifier using the singletube-per-stagecircuit may be employed. The ash-pull circuit is desirable in that itaffords greater power output for the same plate and grid potentials andalso reduces distortion to some extent by eliminating certain harmonicsgenerated in the tubes.

In particular the structure of the amplifier at the distant stationembodies choke coils 7 and 8 and condenser 9 for the puran inputtransformer comprising primary winding 13 and secondary windings 14 and15; two thermionic tubes having grid elements 16 and 17, plate elements18 and 19 and filament or cathode elements 20 and 21 respectively; anoutput transformer comprising primary windings 22 and 23 and secondarwindin'g24; a dynamic speaker 25 inclu ing a voice coil 26 and a fieldcoil 27, which, in the first form, is also employed as a retardationcoil for the purpose of further segregating the input and outputcircuits of the amplifier; a power transformer 28 to supply filamentcurrent to the amplifier and having a primary winding 29 and a secondarywinding 30; and a key 31 available for the use of the subscriber inopening and closing the filament circuit; a relay including winding 32and spring contacts 33 and 34 for the purpose of opening and closing thefilament circuit by a control from the central oflice.

A condenser 35 is interposed between the filament and the platepotential tap on the output transformer for the purpose of furthersegregating the output and input circuits. A tapped resistance 37 isconnected across the filament supply leads and having a center tapconnected to the grid return for the fpurpose of preventing theamplification 0 any alternating current Cross hum which might begenerated in the filament circuit. A grid bias resistance 38 isconnected betweenthe filament and grid return.-

In describing our system, the apparatus and elements utilized will bedescribed in conjunction with the several circuits between the centralstation and the subscriber's home: a

F'Zlawwnt circuit The filament circuit for the thermionic tubes of theamplifier may be conveniently supplied from the household source ofalternating current. This circuit includes winding 30 of the powertransformer, contacts 33 and 34 of relay 32-33-34, filaments 20 and 21and the contacts of key 31.. This circuit is closed and supplies currentto the filaments as long as the relay is operated and the contacts ofkey 31 are closed. Key 31 is available to the use of the subscriberwhile the relay is energized by currents transmitted from the centralstation thereby making it possible for the operators at the centralstation to energize or de-energize the filament circuits of thedistantly located amplifier as may seem desirable. The operation ofrelay 32333-1 will be further treated in the explanation of the controlcircuit.

Grid supply circuit Grid supply potentials are generated for grids 16and 17 by means of potentials created across grid bias resistance 38through a path from positive battery 13 at the central office, contactsof key 6, winding 3 of central ofiice output transformer, line conductorY, choke coil 8, choke coil 27, windings 22 and 23 of the outputtransformer, the space current paths of both tubes, resistance 37.resistance 38, choke coil 7, line conductor X, transformer winding 2 tonegative battery B. This current sets up a voltage across resistance 38which serves to keep the grids 16 and 17 at a negative potential at alltimes in respect to filaments 20 and 21 respectively.

Plate supply circuit Plate potentials and currents are supplied to bothplates through the identical circuit described in the paragraph headedGrid supply circuit.

Signal circuit Alternating potentials generated by the central ofliceamplifier are impressed on winding 1 of the central office outputtransformer causing current to flow through this winding. In turn thiscurrent induces a potential across windings 2 and 3. This potentialcauses current to flow from point I shown on the drawings through lineconductor X, condenser 12, potentiometer winding 10, line conductor Y,to the point Q Reference shown on the drawings. This current in turncreates a potential along the length of potentiometer winding 10 andaccordingly generates a proportional potential between tap 11 and lineconductor Y at point N. This potential causes a current to flow from tap11 through winding 13 to oint N inducing a corresponding potentia ofgreater magnitude across windings 14 and 15, which is applied directlyto grids 16 and 17. This potential is augmented by the normal amplifyingaction of the tubes and the resultmg potential created between plates 18and 19 causes current to flow throu h windings 22 and 23 of the outputtrans ormer. By induction a current is thereby caused to circulatethrough the secondary winding 24 of the output transformer and the voicecoil 26 of the d namic s eaker.

It wil be note that this circuit embodies certain impedances which arecommon both to the input and the output circuit of the amplifier.Specifically line conductors X and Y are used for the transmission ofoperating potentials to the plate circuit and are also used for thetransmission of alternating potentials to the input circuit. For thisrea-- son it is necessary to incorporate means in the device forpreventing the return of alternating currents generated in the outputcircuit to the input circuit. In the case of a push-pull amplifier ofthis type, potentials in the plate circuit to which the supply circuitis accessible are very small, and are lim ited to small unbalancepotentials generated between the supply tap on windings 22 and 23 andthe filament, due to differences in the electrical properties ofwindings 22 and 23 or due to a dissimilarity in the amplifyingproperties of the tubes. These potentials, however, exist to someextent, and if a path of low impedance were provided along the supplycircuit, a certain amount of unbalance current would flow from thesupply tap on windings 22 and 23 through line conductor Y, windings 2and 3 of the central office output transformer, line conductor X andwinding 13 of the input transformer, inducing a corresponding potentialon grids 16 and 17 and cause undesirable regeneration effects.

In order to prevent effects of this kind, means are provided in thisamplifier for maintaining the impedance of that part of the supplycircuit which is disposed between the input and the output circuits ofthe amplifier at a high value in respect to alternating currents. clude:the combination of condenser 9 and choke coils 7 and 8 comprising aconventional electrical filter, and choke coil 27.

In the first form the choke coil 27 is shown to be a part of the dynamicspeaker, a feature which will contribute considerably to economy in themanufacture of the appara- Manner Specifically these means intus.Preferably this choke coil will be the field coil of the speaker and itsimpedance is such that it will retard to a very great extent the passageof any currents due to unbalance potcutials set u between the supply tapon windings 22 an 23 and the filaments of the tubes.

In the filter combination 78-9 the impedance of condenser 9 has a lowvalue in respect to alternating currents and oflers a path whereby anyunbalance currents may readily return to the filament of the tubeswithout traversing the line conductors or the input circuit. On thecontrary, choke coils 7 and 8 ofier an extremely high impedance tounbalance currents and according- 1y prevent the transmission of anysuch currents to the line or input circuit. At the same time, a path ofrelatively low im dance is supplied by choke coils 7, 8 an '27 for thetransmission of direct currents to the plate circuits of the tubes whilecondenser 9 has an infinite impedance for currents of this t pe andaccordingly does not provide a tfi between line conductors X and Y.

n combination, the two elements referred to provide means whereby directcurrents for the purpose of maintaining the various elements of thetubes at operative potentials may be transmitted to the tubes w1thslight loss; whereby alternating currents generated in the plate circuitof the amplifier are prevented from returning to the grid circuit of theamplifier, thereby preventing undesirable regeneration effects; andwhereby signal currents transmitted to the amplifier or line conductorsX and Y are allowed to traverse the input circuit of the amplifierwithout attenuation caused by the various taps taken oil for the supplycircuit.

' Condenser 12 is interposed between the filter 7-8-9 and thepotentiometer 10 for the urpose of preventing1 the passage of supp ycurrents through t e potentiometer winding while at the same timeallowing the passage of alternating signal currents to the input circuitof the am lifier.

Condenser is provi ed at the central office for the purpose of allowingthe passage of signal currents through windings 2 and 3 of the outputtransformer and for the purpose of preventing the passage of supplycurrents from the central ofiice battery B.

Control relay circuit The control relay is energized by currenttransmit-ted from positive central ofiice battery B through contacts ofkey 6, winding 3, line conductor Y, choke coil 8, winding 32 of thecontrol relay, choke coil 7, line con- Volume control operation Asdiscussed briefly in the description of the signal circuit, signalcurrents traversing winding of the potentiometer set u a potentialbetween tap 11 and point N. e magnitude of this potential is directlyproportional to the resistance included between tap 11 and point N.Accordin 1y, as ta 11 is moved along the length 0 the win ing to includea larger amount of resistance, the potential impressed across winding 13is increased. Conversely, if tap 11 is moved to include a smaller amountof resistance, the potential impressed across winding 13 is decreased. Acorresponding and proportional variation in potential is producedbetween grids 16 and 17.

Referring to the second form of the invcntion illustrated in Fig. 2, theap aratus included in this form is similar to t at described inconnection with the first form and the individual elements are disposedsimilarly in respect to each other with the exception that an inputtransformer and potentiometer, each having two windings separated by acondenser, are employed; and that the plate circuit retardation coildoes not form a part of the speaker. The Pl'i'. mary windings of theinput transformer are numbered 7a and 9a and the windings of thepotentiometer are numbered 10a and 11a. The condensers employed inconnection with the transformer winding and potentiometer are numberedrespectively 8a and 12a. \Vith these exceptions the elements of thecircuit are designated by the same numbers in the drawings as wereemployed in the case of the first form.

In describing this form, only such circuits will be treated in detail-asdeviate from the corresponding circuits outlined in the description ofthe first form.

Filament circuit This circuit is similar to the first form.

Grid supply circuit Potentials for maintaining the grids at negativepotentials with respect to the filaments are created between theterminals of resistance 38 due to the normal plate supply current whichflows through a path from positive central otlice battery B, contacts ofkey 6, transformer winding 3, line conductor Y, potentiometer winding11a, transformer winding 90, choke coil 27a, transformer windings 22 and23, the space current paths of both tubes, resistance 37, resistance 38,transformer winding 7a, potentiometer winding 100:, line conductor X,transformer winding 2 to negative central ofiice battery B.

Plate supply circuit Current supply for the plate circuits of the tubesis transmitted through the same tials are transmitted from the centralstation to the remote station along line conductors X and Y in a mannerexactly similar to that described in connection with the first form. Atthe remote station signal potentials impressed on. the potentiometerwindings 10a and 11a cause alternating current to flow through thesewindings and through condenser 12a and set up between any two points onwindings 10a and 11a alternating potentials proportional to the resistance included between the two points. The alternating potentialsbetween potentiometer taps'M and N cause current to How throughtransformer windings 7a and 9a and condenser 8a. By induction acorresponding potential is generated in the secondary windings 14 and 15and impressed on grids l6 and 17 This potential is amplified anddelivered to the loud speaker in a manner similar to the first form.

In this form the combination including transformer windings 7a and 9aand condenser 8a perform a dual function. This combination takes theplace of filter 789 shown in the first form and is effective insegregating the output circuit from the in put circuit. In additionwindings 7a and 9a comprise the primary of the input transformer.Cons1dering this combination of elements as a filter, windings 7a and 9aprovide a path of high impedance to unbalance potentials generated inthe plate circuit thereby preventing the return of any unbalancecurrents to the grid circuit; while at the same time a path of lowimpedance is provided for supply currents. Condenser 8a provides a pathof high impedance to supply currents but allows the passage ofalternating signal currents received trom the line conductors.Transformer windings 7a and 9a and potentiometer windings 10a and 11aare balanced in respect to the line in such a way that any smallcapacity effects existing between elements of the amplifier and groundwill not produce an unbalanced condition of the line.

In this form the plate circuit retardation coil 27a is shown as aseparate element and does not form a part of the speaker, which in thiscase is of the magnetic type.

Control relay circuit The control relay is energized by currentstransmitted from positive central ofiice battery B through contacts ofkey 6, trans- Cross Reference central oflice battery B. By operating key6 relay 32-33-34 may be energized or deenergized, opening and closingthe filament circuit.

Volume control operation As outlined previously in the discussion of thesignal circuit, alternating potentials received from the line causecurrent to flow through the potentiometer windings 10a and 11a andcondenser 12a, settin up between any two points along the lengt l ofwinding 10a and 11a alternating potentials proportional to theresistance included between the two points. Taps are taken 01f fromwindings 10a. and 11a in such a way that as the, potentiometer arms aremoved a connection is made from taps M and N to difi'erent points alongthe length of windings 10a and. 114, such that any increase in theresistance included between tap M and condenser 120 will be accompaniedby a corres onding change in the resistance included etween tap N andcondenser 12a. Accordingly as the potentiometer arms are moved toinclude a greater resistance between taps M and N, a greater potentialis impressed across the input transformer; while as the potentiome-' terarms are moved to include a less resistance a smaller potential isapplied to the,

input transformer.

Referring to the third form of the invention disclosed in Fig. 3, theapparatus included in this form is similar to the first form and thedisposition of the various elements in respect to each other is similar.to the arrangement of the first form with the exception that filtercombination 789 shown in he first form is replaced by the. combinationincluding transformer windings 7b and 8b and condenser 96; and thatpotentiometer 10l1 shown in the first form is replaced by potentiometer10b11b. With these exceptions the elements of the circuit are designatedby the same numbers as were applied to the corresponding elements in thefirst form.

In describing the operation of this form of the invention only suchcircuits will be treated in detail as deviate from the correspondingcircuits of the first form.

" Filament circuit This circuit is similar to the first form. Gridsupply circuit Grid supply potentials for maintaining the grid elementsof the tubes at a negative bias in respect to the filaments are createdbetween the terminals of resistance 38 due to the passage of the platesupply current which flows through a path from positive central ofiicebattery B, contacts of key 6, transformer winding 3, line conductor Y,transformer winding 86, retardation coil 27, transformer windings 22 and23, the space current paths of both tubes, resistance 37, resistance 38,transformer winding 76, line conductor X, transformer winding 2, tonegative central oflice battery B.

. Plate supply circuit tentiometer windings 10b and 11b and ac cordinglyset u between any two points along the lengt of windings 10b and 11balternating signal potentials directly proportional to the resistanceincluded between the oints. These potentials are applied direct y togrids 16 and 17, and are amplified and delivered to the loud speaker ina manner similar to that described in connection with the first form.

As in the case of the previously described second form, the combinationincluding transformer windings 7'6 and 8?) and condenser 9b performs adual purpose in acting as a segregatin unit for the se aration of theinput and output circuits an also in forming a part of the input circuitfor the delivery of the signal current to the grids of the tubes.

Control relay circuit This circuit is similar to the first form.

Volume control operation The operation and construction of the volumecontrol are similar to those described in connection with the previouslyoutlined second form, except that the potentials generated between tapsM and N are ap lied directly to grids 16 and 17 and are not elivered' tothe tubes through the input transformer.

From the foregoing description it will be seen that the threeembodiments of the invention disclosed successfully fulfill the objec'tsset forth. Of the three forms disclosed, the form shown in Fig. 1presents certain advantages in relation to the other g forms, but eachof the other forms possesses lesser points of advantage in certainremeans tion can be made to have practically anyv constants which may bedesirable in order to produce the most effective segregating properties.When the filter employed is a part of the input transformer and whenthis input transformer constitutes the line termination, the impedanceof the retardation coil is limited by the necessity of making theimpedance of both coils in series e ual to the characteristic impedanceof the ine. This disadvantage is overcome by usii a separate filter. Asecond advantage 0 the first form is that no supply current traversesthe windings of the potentiometer effecting substantial economies in then'ianufacture of this element. A third advantage exists in the fact thatno supply current is allowed to traverse winding 13 of the inputtransformer thereby preventing undesirable saturation efl'ects.

The form of the invention disclosed in F igi 2 has the advantage thatthe separate filter combination is eliminated with a corre-' spondingdecrease in the cost of mannfac ture. Adequate segregatingcharacteristics in the transformer windings 7a and 9a are maintained byplacing the volume control on the line side of the transformer therebymaintaining the impedance of the transformer windin at a relatively highvalue.

The form 0 the invention disclosed in Fig. 3 has the same advantages asFig. 2 in relation to the elimination of the separate filter and has theadditional advantage that no current is allowed to traverse thepotentiometer windings enabling the construction ofthis element at alower cost.

Figs. 1 and 3 show the use of the field coil of the dynamic speaker as aretardation coil for the purpose of impeding the flow of unbalancecurrents from the plate circuit to the input circuit. This featureshould result in desirable savings in manufacturing costs.

In the three embodiments of the invention disclosed it is to be borne inmind that the impedance ratio of the central oifice output transformeris such that the impedance of the source of signal current 4 isreflected through to the line at a value equal to the characteristicimpedance of the line. It is a well known principle in transmission ofalternating currents that under these conditions a mlnimum voltage levelis re ired at the central station for the transmission of a maximumvoltage level to the remote station. For this reason in the threeenibodiments of the invention disclosed the signal currents will betransmitted from the central station to the remote station at a voltagelevel sufiiciently low to obviate the possibility of causing any crosstalk" in neighboring conductors in the same communication cable, itbeing remembered that our system is especially adapted for use inprogram service sup lied by telephone companies wherein con uctors inconventional communication cables may be utilized for the transmissionof sound or image-carrying currents as well as for the transmission ofoperating electro-motive forces for the remotely located amplifiers.

In the appended claims the term alternating currents is meant to includeall currents described in the'foregoing specification as sound-carrying,image-carrying or signal currents. The term space current path is meantto indicate the path of conductivity existing in a conventional vacuumtube between the plate and filament due to the flow of electrons betweensaid two elements.

What is claimed is:

1. In transmission systems for alternating current, a central stationhaving available a source of direct current and a source of alternatingcurrent, a distantly located station, a thermionic am lifier at saiddistant station having grid, p ate, and cathode elements and includinginput elements, a pair of metallic conductors extending between saidcentral station and said distant station, a circuit for said alternatingcurrent including said conductors and including said input elements, acircuit for said direct current also including said conductors andincluding the space current path of said amplifier, a resistance elementat said distant station inserted in said direct current circuit formaintaining said grid element at operative bias potential with respectto said cathode element, a relay at said distant station having anactuatin coil and a pair of contacts which are close when said coil isenergized, and a local source of electrical energy at said distantstation for heating said cathode element, the coil of said relay beinconnected to said direct current circuit and the contacts of said relaybeing inserted in a circuit connecting said local source to said cathodeelement.

2. In transmission systems for alternating current, a central stationhaving available a source of direct current and a source of alternatingcurrent, a distantly located station, a thermionic amplifier at saiddistant station having grid, filament, and plate elements and includinginput elements, a pair of metallic conductors extending between saidcentral station and said distant station, a circuit for said alternatingcurrent including said conductors and including said input elements, acircuit for said direct current also including said conductors andincluding the space current path of said amplifier. a coil at saidcentral station coupled to said source of alternating currents, saidcoil being divided at its midpoint to form two equal sections thereof,said source of direct current being connected in series with and betweensaid sections, a condenser connected in multiple with said source ofdirect current, the remainin terminal of said coil being connected to te respective conductors of said pair.

3. In transmission systems for alternating current, a central stationhaving available a source of direct current and a source of alternatingcurrent, a. distantly located station, a thermionic amplifier at saiddistant station having "rid, filament, and plate elements and includinginput elements, a pair of metallic conductors extending between saidcentral station and said distant station, coupling means at said centralstation connecting said source of alternating current and said source ofdirect current tosaid conductors, a coil at said distant station havingtwo sections, a condenser connected in series with and between" saidsections and the remaining ends of said sections bein connected to therespective conductors 0% said pair, said input elements being connectedto said conductors, a condenser inserted in series with said inputelements, the space current circuit of said amplifier being connected inparallel with said first mentioned condenser, a source of electricalenergy local to said distant station for heating said filament element,a relay having an actuating coil and having electrical contactsconnected in series withgaid local source, the coil of said relay beingconnected in parallel with said first mentioned condenser.

4. The structure defined in claim 3 and a switch at said central stationconnected in series with said source of direct current whereby thesupply of direct current to the space current circuit of said amplifierand to said relay coil may be simultaneously controlled.

5. In transmission systems for alternating current, a central stationhaving available a source of direct current and a source of alternatingcurrent, a distantly located station, a thermionic amplifier at saiddistant station having grid, filament, and plate elements and includinginput elements, a pair of metallic conductors extending between saidcentral station and said distant station, a coil at said central stationcoupled to said source of alternating circuits, said coil being dividedinto'two equal sections, said source of direct current being connectedin series with and between said sections, a condenser connected inparallel with said source of direct current, the remaining terminals ofsaid coil being connected to the respective conductors of said pair, acoil at said distant station having two sections, a condenser connectedin series with and between said sections and the remaining ends of saidsections being connected to the respective conductors of said pair, saidinput elements being connected to said conductors, a condenser insertedin serice with said input elements, the space current circuit of saidamplifier being connected in parallel with said second mentionedcondenser, a source of electrical energy 10- cal to said distant stationfor heating said filament element, a relay having an actuating coil andhaving electrical contacts connected in series with said local source,the coil of said relay being connected in parallel with said firstmentioned condenser.

In testimony whereof we aflix our signatures.

DONALD B. HARRIS. O. W. KNAUSS.

meme

